Americans spend more than $6 billion a year on gluten-free foods, thanks in part to the products' health halo. But are all those potato-flour pastas and pizzas getting to the right stomachs? According to a Mayo Clinic study, 1.6 million Americans who are on gluten-free diets don't need to be, while the vast majority of the 1.8 million who could benefit aren't aware they should watch their intake.

So who really needs to go gluten free? Only those with the following conditions:

Celiac Disease Because its symptoms are so varied, this autoimmune problem can be tricky to identify. In sufferers, the immune system launches an attack when it detects gluten (the protein in wheat, rye, and barley) in the digestive tract, damaging the small intestine in the process. The result: gastric distress and poor absorption of nutrients, potentially leading to anemia, osteoporosis, hair loss, and even cancer and infertility. Celiac disease is usually diagnosed based on symptoms, plus blood tests for autoantibodies, and then confirmed with a biopsy of the small intestine.

Gluten sensitivity Short of celiac disease, gluten sensitivity can cause headaches, grogginess, and fatigue. If you have any of those symptoms, plus a stomachache after eating wheat, see your doctor. But don't cut bread out of your diet just yet, or you'll eliminate your No. 1 diagnostic tool, says Alessio Fasano, MD, director of the Center for Celiac Research at the University of Maryland.